This invention relates to a method and an apparatus for checking value documents, such as e.g. bank notes, checks, cards, tickets, coupons.
From the prior art it is known to equip value documents with security elements, such as security strips or also security threads, which contain magnetic material. The magnetic material can be applied to the security element either continuously or only in some regions, for example in the form of a coding. For magnetically coding a security element there is used for example a certain sequence of magnetic and non-magnetic regions that is characteristic of the value document. Moreover, it is known to employ different magnetic materials for a magnetic coding, e.g. with different coercive field strengths. For example, there are used two different-coercitivity magnetic materials from which two sorts of magnetic regions are formed which are disposed on the security element.
Further, it is known to machine check bank notes having security threads which have a magnetic coding consisting of different-coercitivity materials. The bank notes are transported parallel to the longitudinal direction of the security element, successively first passing through a strong magnetic field parallel to the transport direction, which magnetizes the high—as well as the low-coercivity magnetic regions along the transport direction. The remaining magnetization is checked by means of an inductive magnetic detector which is sensitive parallel to the transport direction. Subsequently, the bank notes pass through a weaker magnetic field perpendicular to the transport direction, which aligns only the low-coercivity magnetic regions perpendicular to the transport direction, while the high-coercivity magnetic regions remain magnetized in the transport direction. Again, the remaining magnetization is checked by means of an inductive magnetic detector which is sensitive parallel to the transport direction. The first inductive magnetic detector thereby detects the high- and the low-coercivity magnetic regions, and the second inductive magnetic detector detects only the high-coercivity magnetic regions. However, if the security element also contains combined magnetic regions containing both different-coercitivity magnetic materials, so that the different-coercitivity magnetic materials pass into the detection region of the magnetic detector at the same time, a superimposition of the magnetic signals of the different-coercitivity magnetic materials is detected. The combined magnetic regions thereby deliver a reduced magnetic signal whose signal swing lies between that of the high-coercivity and that of the low-coercivity magnetic regions. It is disadvantageous in this method that these combined magnetic regions are difficult to distinguish from the high-coercivity and from the low-coercivity magnetic regions. Furthermore, it is disadvantageous to employ, for remagnetizing the low-coercivity magnetic regions, a magnetic field that is oriented parallel to the transport plane and perpendicular to the transport direction of the value document. For, using conventional magnets, thus oriented magnetic fields can only be realized with relatively low magnetic field strength.